1. Field of the Invention
The present invention relates to a method for purifying a crude 2,4-xylenol mixture containing 2,5xylenol, more specifically, to a method for producing 2,4-xylenol of high purity from crude 2,4-xylenol containing 2,5-xylenol.
2,4-Xylenol is useful as the starting material for synthetic resins, adhesives, insecticides, antioxidants, herbicides, dyes, etc. and has been produced and used in great amounts. Depending on the specific end use, products of high purity are required.
2. Description of the Prior Art
Xylenol includes six isomers, that is, 2,3-xylenol, 2,4-xylenol, 2,5-xylenol, 2,6-xylenol, 3,4-xylenol and 3,5-xylenol. Since the boiling points of these isomers are close to each other, it is extremely difficult to efficiently separate the respective isomers from a mixture thereof by distillation. Particularly, since the boiling points of 2,4-xylenol and 2,5-xylenol are substantially identical, they cannot be separated by mere rectification. Consequently, a 2,4-xylenol with a purity of about 90-95% at the highest is the purest commercially available at present.
Various methods of separating 2,4-xylenol and 2,5-xylenol have been proposed. For instance, one separating and purifying method utilizes the fact that 2,4-xylenol forms a molecular compound with methyl quinoline, aniline, toluidine, etc. (for example, see U.S. Pat. No. 2,526,807); another involves alkylation into a tert-butylated product by reaction with isobutylene, separating the tert-butylated product by fractional distillation and then carrying out dealkylation in the presence of an acid (for example, see British Patent No. 582,057); another separating method involves sulfonation and, successively, carrying out partial hydrolysis with super heated steam (for example, see U.S. Pat. No. 2,327,312); and another method uses an adsorbent, such as a molecular sieve, calcium oxide or zeolite.
Another method involves reacting a mixture of xylenol isomers with formaldehyde in the presence of a strong acid, such as hydrochloric acid, or a strong base such as sodium hydroxide and then separating only the 2,5-xylenol in the form of a xylenol-formalin resin (for example, see U.S. Pat. No. 2,917,487).
U.S. Pat. No. 2,917,487 discloses conducted the condensation reaction at temperatures of 50.degree.-100.degree. C. using hydrochloric acid as a catalyst in an amount of 2-8% by weight to obtain purified 2,4-xylenol with a purity of 95.1% at the highest and a purity-converted yield of 67.3%. Such purity and yield are not fully satisfactory, particularly because, depending on contemplated use of 2,4-xylenol, purification products of a purity higher than 95% and, desirably, from 98 to 99% may at times be needed. 2,4-Xylenol of such a high purity cannot be obtained by the method of the above-cited U.S. patent without reducing the yield of 2,4-xylenol to an unacceptably low level. In addition, according to this method, after selectively condensing only the 2,5-xylenol with formalin in the presence of the acid catalyst, the reaction mass is distilled under heating to separate the 2,4-xylenol and the acid used for the condensation reaction is removed by water washing. However, the removal of hydrochloric acid with water washing causes a wastewater treatment problem at an industrial level. This problem can be overcome by removing the hydrochloric acid through neutralization by adding an alkali such as sodium hydroxide to the reaction mass after the condensation reaction. However, in this case, since the reaction mass containing the alkali metal is heated, care must be taken in view of the handling safety. Further, the loss in 2,4-xylenol is not negligible.
For the foregoing reasons, it is presently very difficult to obtain 2,4-xylenol with high purity in industrial amounts.